The present invention generally relates to satellite ephemeris and attitude determination. More specifically, the present invention relates to utilizing communications link pointing information and knowledge of the ephemeris of other satellites to determine satellite ephemeris and attitude.
Attitude and orbit determination and control are important aspects of most satellite systems. The satellite Attitude Determination and Control Subsystem (hereinafter "ADCS") measures and maintains the spacecraft's attitude (or orientation about its center of mass). The ADCS stabilizes the vehicle and orients it in desired directions during the mission despite the external disturbance torques acting on it. This first requires the spacecraft to determine its attitude using sensors. External references must be used to determine the absolute attitude of the spacecraft. The external references may include the Sun, the Earth's infrared horizon, magnetic fields, and the stars. To maintain attitude reference between calculations based on external references, the satellite may carry inertial sensors such as gyroscopes. Next, the ADCS must control the spacecraft's attitude using actuators, such as reaction wheels, control-moment gyros, magnetic torquers, and gas jets or thrusters.
Similarly, the guidance and navigation function, also known as the Orbit Determination and Control Subsystem (hereinafter "ODCS"), measures and controls the position of the spacecraft's center of mass. The position (and optimally the velocity) of a spacecraft as a function of time is commonly referred to as the satellite ephemeris. The ODCS determines the spacecraft's position in space using sensors such as GPS receivers. External references must be used to determine the absolute position of the spacecraft. The external references may include the Sun and the Earth's infrared horizon with precision clocks, or ground reference points and precision clocks. Next, the ODCS must control the orbital position of the spacecraft using gas jets or thrusters. Orbital control is required whenever a satellite is trying to maintain a desired orbit. Orbital control is needed to overcome orbit perturbations and may be required for any orbit, including Low-Earth Orbit (hereinafter "LEO") and geosynchronous Earth Orbit (hereinafter "GEO") stationkeeping. Maintaining relative satellite positions, such as in constellation maintenance, also requires orbital control.
Navigation provides the information necessary to determine a satellite's ephemeris, just as attitude determination provides the information necessary for attitude control. Guidance refers to the means of adjusting a satellites position. Thus, a requirement for orbit control will ordinarily result in a corresponding requirement for guidance and navigation. Ephemeris information may also be used in processing data from the payload. Irrespective of orbit control, there is often a need to point an antenna or instrument in some direction to perform communication or observation tasks. For example, in satellite systems designed to track objects, knowledge of the positions of the satellites sensing the objects being tracked may be critical. Since the position of an object being tracked is ultimately derived from the position in space of the satellite(s) sensing the object, the accuracy of the tracking is directly dependent upon the accuracy with which the position of the satellite(s) is known.
Forces act on the satellite to continually move it away from the nominal attitude and orbit. There are short-term orbital variations (also know as "perturbations") that are periodic with a period less than or equal to the orbital period, and there are long-period perturbations, which are orbital variations with a period greater than the orbital period. There are also secular variations, which represent a linear orbital variation that increases over time. The primary forces that perturb a satellite orbit arise from third bodies such as the Sun and the Moon, the non-spherical mass distribution of the Earth, atmospheric drag, and solar radiation pressure.
In the past, guidance and navigation have involved intense ground-operation activity. However, on-board computers have become computationally powerful, lightweight, and energy efficient. Satellites now carry advanced on-board computers and are capable of performing autonomous guidance and navigation. Another important factor enabling a move to autonomous navigation is the development of accurate on-board sensors, such as GPS receivers. The principal problem remaining is that of providing the on-board computers with ephemeris and attitude data from alternate sources that have high reliability and with improved economic in terms of both cost and weight.
Many autonomous navigation methods currently exist. For example, the Microcosm Autonomous Navigation System uses observations of the Earth, Sun, and Moon, and determines orbit, attitude, ground look point, and Sun direction. Its typically accuracy is approximately 100 m-400 m in a LEO system. Another navigational aid is the Space Sextant, which uses the angle between particular stars and the Moon's limb. The space sectant determines both orbit and attitude, and its typical accuracy is 250 m. Stellar refraction is another navigation system and uses the refraction of starlight passing through the atmosphere to determine both orbit and attitude. Its typical accuracy is 150 m-100 m. Yet another system is Landmark Tracking, which makes use of angular measurements of landmarks to determine both orbit and attitude. Its typical accuracy is measured in kilometers.
The most popular navigational system is Navstar, also known as the Global Positioning System (hereinafter "GPS"), which uses a network of navigation satellites. GPS is currently operational and can provide spacecraft ephemeris information for orbit determination via on-board GPS receivers and GPS receiver antennas. Attitude determination using GPS and multiple GPS antennas has also been demonstrated. The positional accuracy obtainable from GPS is in the 15 m to 100 m range depending on whether the system is using military or commercial grade data. GPS receivers receive signals from multiple GPS satellites and use the received information to solve simultaneously for the three components of the observer's position and the current time. This information is continually updated, providing position and velocity information, which is in turn used to determine orbital parameters. The GPS constellation is at approximately half-geosynchronous altitude and works best for LEO satellites. Since GPS is operationally proven and at least as accurate as other known navigational systems, it is commonly used.
However, two significant problems with GPS are reliability and cost. The potential lack of availability of the GPS satellites for even a short period due to either geometrical circumstances, the failure of one of more of the GPS satellites, or the failure of the on-board GPS receiver is a major concern for an expensive spacecraft which depends on GPS for attitude and positional determination. In addition, space grade GPS receivers can be prohibitively expensive, particularly since satellite systems in need of accurate positional data generally employ redundant GPS receivers on-board each satellite. The additional expense of a redundant GPS receiver, particularly to each satellite in a satellite constellation potentially comprising dozens of satellites with limited lifespans, is substantial. One motivation for the current invention is to provide a method and apparatus which would effectively serve as a backup to an expensive primary navigational system such as GPS.
A need has long existed for an improved satellite attitude and ephemeris determination system.